The present invention relates to a process for dry recycling of (U,Pu)O2 mixed-oxide nuclear fuel scraps.
The manufacture of fuel for light water reactors, based on uranium and plutonium oxide, generally called MOX fuel, has been the subject of various developments connected to the desire to recycle plutonium recovered during the reprocessing of spent fuel.
The manufacture and the irradiation of MOX fuel in light water reactors are now considered as a solution for giving acceptable resistance to the proliferation of plutonium separated from fission products, whether the plutonium is of civil or military origin.
Many processes for manufacturing MOX fuel have been developed during the last two decades, some calling for the complete milling of UO2 and of PuO2 powders in order to provide an intimate mixture, others being limited to milling only a fraction of these powders.
The MIMAS (Micronization and MASter blend) process, which was developed by the applicant of the present invention (see FIG. 1), carries out the micronization by milling only a fraction of the final blend and uses two successive blending operations to allow isotopic homogenization and to take advantage of the use of free-flowing UO2 feed products. The use of free-flowing UO2 in the second blend and the limitation of milling to the first blend alone simplify the manufacture (for example by avoiding prior compacting/granulation or spheroidizing operations on the mixed oxide blend) and have considerably simplified qualification of the MOX fuel by users and licensing by the nuclear safety authorities, at the start of its industrialization (by virtue of the similarity in behavior of this MOX fuel and of the UO2 fuel).
While the mixed oxide fuel is being manufactured for light water reactors, large quantities of scraps are produced during development of the manufacturing process and continue to be produced during routine manufacture; these quantities of scrap are connected to the process itself, to the fuel user specifications, to the traceability of the products (batch production) and to monitoring their quality by sampling.
Processes for treatment of mixed oxide scraps by liquid routes are known. These processes have various considerable drawbacks: on the one hand, they generate considerable liquid effluents and additional criticality risks; on the other hand, they require additional packaging and transport in the frequent case where the liquid route treatment plant is not located on the same site as that of scrap production.
There is therefore a need to be able to directly dry-recycle manufacturing scraps of this type, at the location of their production, in the manufactured fuel.
In addition, experience has shown that dry recycling of scraps without particular precautions can lead to product defects during pellet manufacture namely excessive variability of the physical characteristics of the product, differential-shrinkage defects (for example connected to the direct recycling of the grinding powders), blister defects caused by volatile impurities, etc. Generally, the production of a product with controlled specifications involves controlling the characteristics of the input products.
In order to solve the drawbacks mentioned above, the recycling process of the invention comprises:
a process for manufacturing (U,PU)O2 mixed oxide fuel pellets including:
a dispensing and a first blending of scraps in powder form and, if required, of PuO2 and/or UO2 powders,
micronization and forced sieving of said first blend,
another dispensing and a second blending of the first sieved blend, of UO2 powders and, if required, of scraps in powder form,
pelletizing of the second blend, and
sintering of the resulting pellets, and
a process for pretreating scrap including:
pelletizing and sintering of powder scraps in order to form scrap pellets, and
micronization of the scrap pellets in order to form scrap powder designed to be incorporated as scraps in powder form into the first and/or second blends.
In this way, a process for dry-recycling the manufacturing scraps in the MOX fuel is obtained, and this process can deal with the integral quantity of scraps. This process can also be used to recycle (U,Pu)O2 mixed oxide fuel scrapped due to shortage or discontinuation of its utilization.
According to one embodiment of the invention, scrap unsintered powders (for example, end-of-batch powders from pelletizing) and/or powders arising from grinding of fuel pellets are taken as powder scraps for the aforementioned pelletizing and sintering pretreatment.
According to one advantageous embodiment of the invention, up to 40% by mass of scrap, with respect to the net production of pellets, is incorporated into the aforementioned process for manufacturing fuel pellets.